Spray-drying is the standard method for manufacturing laundry detergent base powder. Typically, detergent ingredients are mixed together to form an aqueous detergent slurry in a mixer, such as a crutcher mixer. This slurry is then transferred along a pipe through a first low pressure pump and then through a second high pressure pump to a spray nozzle, and the slurry is sprayed into a spray-drying tower, and spray-dried to form a spray-dried powder.
The low pressure pump needs to be positioned as close to the mixer, such as crutcher mixer, as possible. This is to avoid starvation of the low pressure pump and to ensure consistent feed to the low pressure pump. However, it is also desirable to limit the length of the portion of the pipe that needs to handle the outlet pressure from the high pressure pump due to the cost and complexity of the high pressure capable pipe work. In other words, it is desirable to position the high pressure pump as close as possible to the spray nozzle. This means that typically there is a need to have a significant distance between the low pressure pump and the high pressure pump. However, this significant distance results in a significant pressure drop along the portion of the pipe between the two pumps. This is problematic because the high pressure pump has a minimum inlet feed pressure to avoid starvation of the high pressure pump, so one is constraint by how far apart (i.e. the length of connecting pipe) these two pumps can be. This in turns results in a dichotomy of process requirements.
The inventors have overcome this problem by introducing a stream of detergent ingredient, especially viscosity increasing detergent ingredients such as alkyl benzene sulphonic acid, into the pipe between the low pressure pump and high pressure pump, and carefully controlling its flow rate and point of addition so as to control the pressure drop between the two pumps. This minimizes the pressure drop between the two pumps, enabling the two pumps to be placed further apart (i.e. allowing the length of the connecting pipe between the two pumps to be increased) whilst maintaining efficient operation of the pumps.